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Quelques réflexions sur le complexe basique-ultrabasique de la zone d'Ivrée et les ultramafites alpinotypes

ContributorsVuagnat, Marc
Publication date1968

Two groups of ultramafic rock can be recognized in the Western Alps. 1. Ultramafics, mostly serpentinites, of the ophiolites sequence. These rocks have been emplaced during the alpine orogenesis in the region of the Pennine Nappes. Some are important massifs of more or less serpentinised peridotites in the root-zone of the Pennides; others of lesser volume seem to be pieces of similar bodies which have been fragmented. These fragments have been dislocated, sometimes completely serpentinised and tectonically emplaced in more external and higher parts of the alpine frame. These ultramafics can be considered as alpinotype although they do not show all the features proposed by Thayer for alpine complexes. 2. Ultramafics of pre-Alpine basement series. The Ivrea Zone ultramafics, with the Finero peridotite as the major body, enter into this category. This body has been considered by some to be an intrusion of peridotitic magma into the Ivrea zone during the alpine orogenesis and it would thus be part of the "stem" of the alpine ophiolites. However the great freshness of the olivine crystals, the occurrence of phlogopite in the core, of garnets in the transition terms to basic rocks, the strongly oriented textures and the palaeozoic age given by a K/Ar determination distinguish the Finero peridotite from the ophiolite ultramafics. If this big body is pre-Alpine several hypotheses can be proposed; it could be part of an ophiolitic complex belonging to an older orogeny and having suffered intense metamorphism or a layered intrusive in an old basement or a fragment of the upper mantle. The lack of statistical geochemical data however precludes the making of a rational choice between these hypotheses although the last one appears slightly more probable. It could be that we have here a piece of upper mantle "pristine" material having escaped the partial fusion linked to the generation of basic magmas, hence its high alkalis content. Finally, it must be kept in mind that if we admit that the greater part of alpinotype ultramafics are mechanically emplaced fragments of the upper mantle, this does not preclude the possibility that some of them were parts of ultramafic complexes in portions of an old basement, which has been "reworked" by a new orogenesis in a kind of tectonical rejuvenation.

Citation (ISO format)
VUAGNAT, Marc. Quelques réflexions sur le complexe basique-ultrabasique de la zone d’Ivrée et les ultramafites alpinotypes. In: Schweizerische mineralogische und petrographische Mitteilungen, 1968, vol. 48, n° 1, p. 157–164. doi: 10.5169/seals-37758
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ISSN of the journal0036-7699

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